Chlorpyrifos is popular for farmers because it controls a range of insects that destroy some of the state's key crops.
But environmentalists say it's a toxic air contaminant that can harm brain development in babies. Research shows some of the effects include lowered reading ability and IQ, smaller head size and hyperactivity in children.
It has been used on about 60 different crops in California, including grapes, almonds and oranges. Farmers say in particular it helps eliminates pest that threaten to decimate the state's citrus industry.
It could take up to two years for California to finalize the ban due to legal challenges.